The present invention relates generally to waveform correction, and more specifically to a circuit for cancelling whole or part of a waveform using a digital filtering technique. The invention is particularly useful for cancelling echo returning from a hybrid interface between a four-wire and a two-wire section of a digital transmission system or cancelling intersymbol interference that occurs between successive digital waveforms.
Conventional echo cancellers and decision feedback equalizers for digital transmission employ a nonrecursive filter (or alternatively called, transversal or finite impulse response filter) having a plurality of tap weights which are adaptively controlled in accordance with the waveform of an incoming symbol so that it produces a replica of the undesired waveform to be destructively combined with the incoming symbol. Because of the finite numbers of the tap weights, the nonrecursive filter is not capable of completely cancelling a long tail portion of the undesired waveform which occurs as a result of the low-frequency cutoff characteristic of a line transformer. It is not practical to provide as many tap weights as necessary to cancel the tail of a waveform.